An input level to the first mixer of max -20 dBm (22.4 mV) shall give
approx. 100 mV to an AD606 log.det. for for full deflection on a scope
with 8 vertical divisions and 10dB/Div. This means a gain of 5.

With -20 dBm to the first mixer it is a good idea to have about
the same level to mixer 2 and 3. MAR amplifiers from Mini-Circuits
can be used to compensate for losses in the mixers and filters.

3 dB PAD´s can be distributed to improve matching.

First LO: 284-484 MHz

Obtaining low phase-noise in a VCO is difficult. A factory made VCO
is recommended. The control voltage must be free from noise. With
narrow BW´s phase noise turns up. An RC-network (3 k and 1uF to
ground) at the control input improves matter. A low-noise op-amp
in the sweep-circuit is a good idea.

Powersupply:

Magnetic fields can induce hum-voltages in the VCO-control voltage.
A toroidal mains-transformer is recommended.

A sweep-generator or a network-analyzer is a must for tuning the filters.